For generations municipal solid waste was disposed of by depositing it in landfills. As an understanding of the potential for air and groundwater pollution from landfills developed, landfill technology evolved to include lining the landfill with a substantially impervious boundary and covering the landfill with a permeable or impermeable daily cover. When the landfill reached its capacity, traditionally the landfill would be covered with a substantially impermeable barrier and vertical vents would be installed to release gasses generated in the biodegradation of the landfilled municipal solid waste. Typically the solid waste would degrade anaerobically and the primary gas produced during this anaerobic decomposition was methane. Any biodegradation taking place in the landfill would proceed slowly, meaning it took many years for the landfill to stabilize such that methane production substantially ceased and the landfill reached its maximum settlement.
In the last few decades landfill operators have taken a more active role in promoting biodegradation of the deposited municipal solid waste material. The term “bioreactor landfill” has come into general use in the past decade to denote a landfill that is operated in such a way as to enhance the decomposition of municipal solid waste rather than simply contain it. Recirculation of leachate collected at the bottom of the landfill has been the primary method of enhancing the rate of waste decomposition. Introduction of additional liquids has also been used to increase the landfill moisture content to an optimal level for biodegradation of the organic materials in the municipal solid waste. In recent years, an operational definition of “bioreactor landfill” has become a landfill that adds (or is designed and equipped to add) water in addition to recirculating leachate. The USEPA essentially adopted this definition for bioreactor landfills in its Landfill MACT Rule governing air emission controls at municipal solid waste landfills. (40 CFR Part 63 National Emission Standards for hazardous Air Pollutants: Municipal Solid Waste Landfills, Fed Reg. Vol. 68, No. 11, p. 2227 (Jan. 16, 2003)). The bioreactor landfill concept encompasses both anaerobic and induced aerobic decomposition processes as well as “hybrid” processes in which aerobic conditions are induced initially in order to minimize the acid production phase of a subsequent anaerobic decomposition phase.
A number of patents are directed to hybrid bioreactor landfills and teach that promoting aerobic biodegradation followed by anaerobic biodegradation under controlled conditions can maximize the speed of biodegradation and therefore result in more efficient volume reduction and stabilization of municipal solid wastes. Representative patents are Hater, U.S. Pat. No. 6,283,676; Ham, U.S. Pat. No. 5,984,580; and Hudgins, U.S. Pat. No. 6,364,572.
The Hater patent contains a useful summary of prior art efforts to increase the efficiency of bioreactor landfills. Hater instructs that the prior art focused extensively on leachate recirculation and maintaining a high moisture content in the municipal solid waste. Hater teaches in addition to maintaining a high moisture content in waste, the desirability of adding materials to accelerate the aerobic or anaerobic decomposition of waste material. Representative additives include sludge, animal manure, fermenter byproducts as well as nutrients such as phosphorous, phosphoric acid, biosolids phosphate buffer and the like which may be added either directly to the waste or added to liquids applied to the waste.
Ham, in addition to teaching the desirability of leachate recirculation to promote efficient biodegradation and collection of methane resulting from anaerobic digestion, teaches that the efficiency of biodegradation can be improved by comminuting solid waste particles to an approximately uniform particle size distribution.
Green, U.S. Pat. No. 5,888,022, while directed exclusively to aerobic digestion, teaches the desirability of controlling temperature within the aerobic digester by controlling the rate of airflow through the digester. Green further teaches the desirability of adding nutrients such as nitrogen, phosphates and carbon sources by injection wells to maintain optimal levels of microbial growth for aerobic digestion. Green teaches that maintaining the aerobic digestion in a temperature range of 130°-150° F. (54° to 65° C.) can substantially eliminate pathogens from a landfill.
While the prior art discussed above is generally concerned with optimizing the biodegradation processes in order to more efficiently stabilize and compact municipal solid wastes, the prior art fails to suggest a method which optimizes methane production and accelerates stabilization and compaction of the landfill while minimizing the risk of fire which has plagued prior art hybrid and aerobic digestion techniques. Furthermore, the prior art fails to teach a municipal solid waste landfill system providing a number of bioreactor cells enabling the efficient biodegradation and methane recovery of mixed municipal solid wastes as well as source separated municipal solid wastes. Finally, the prior art fails to provide bioreactor cells which can be efficiently reclaimed for reuse as needed.
The present invention is directed toward overcoming one or more of the problems discussed above.